As a method which permits efficient welding of objects of welding such as thick steel plates with uniform penetration of molten metal without causing welding defects, there is disclosed in Japanese Pat. Provisional Publication No. 133,871/80 an arc-welding method of a narrow groove, which comprises:
directing a nozzle rotatable about the center axis thereof substantially vertically toward an I-shaped narrow groove formed between objects of welding; directing a consumable welding electrode through said nozzle eccentrically from the center axis of said nozzle toward said groove; feeding a welding current to said consumable welding electrode to produce an arc between said groove and the tip of said consumable welding electrode to weld said objects of welding with each other by means of heat from said arc; rotating said nozzle about the center axis thereof at a high speed of at least 120 r.p.m. to cause a circular movement of said arc from the tip of said consumable welding electrode corresponding to the eccentricity thereof; and feeding a shielding gas toward said arc and a weld zone to shield said arc and said weld zone from the open air (hereinafter referred to as the "prior art").
In the prior art described above, the welding current is fed to the consumable welding electrode by a welding current feeding mechanism provided on the tip portion of the nozzle or on a consumable welding electrode feeding mechanism located above the nozzle. However, the welding current feeding mechanism, if provided on the lower end portion of the nozzle, would be required to have a complicated construction because of the high-speed rotation of the nozzle and may be damaged by the arc heat or spatters produced during welding. Furthermore, when the groove width is very small, the welding current feeding mechanism may make it difficult to insert the nozzle into the groove. When the welding current feeding mechanism is provided on the consumable welding electrode feeding mechanism located above the nozzle, on the other hand, the considerable length of the consumable welding electrode between the tip thereof and the consumable welding electrode feeding mechanism leads to a larger electric resistance of the consumable welding electrode, thus resulting in a larger loss of the welding current.
Under such circumstances, there is a strong demand for the devclopment of a rotary arc-welding apparatus which permits sure feeding of a welding current by a simple welding current feeding mechanism to a nozzle rotating about the center axis thereof at a high speed, without causing damages to the welding current feeding mechanism by the arc heat or spatters produced during welding, and gives only a small loss of the welding current, and furthermore, has a welding current feeding mechanism not hindering insertion of the nozzle into the groove even when the groove width is small, but such an apparatus is not as yet proposed.